This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Sir2 enzymes catalyze the deacetylation of acetyl lysine residues. Additionally, some Sir2 enzymes can catalyze the ADP ribosylation of arginine side chains. The reaction mechanism of Sir2 mediated ribosylation is not known. To understand how Sir2 enzymes catalyze ribosylation of arginine, I have trapped a Sir2-ribosylated arginine product in Sir2 crystals in addition to Sir2-acetyl arginine peptide-NAD+, which is the Michaelis complex for ribosylation. In order to obtain high resolution data that is necessary for our mechanistic studies, we propose using the BIOCARS X-Ray beam lines for our studies.